This is a study of changes in protein synthesis as tissue chronologically ages during differentiation in vivo and in vitro. Skeletal muscle is one of several tissues in which the principal specific proteins which characterize the differentiated cell are of isozymic forms that change qualitatively and quantitatively as the organism ages. The regulation of these changes is thought to be mediated both on a genetic and an epigenetic or environmental basis. Skeletal muscle in particular is an excellent system to determine the interplay of genetic and epigenetic factors in changes of specific isozymes related to developmental age. By the study of isozymic transitions of myosin as cells age from embryonic to adult stage, the mechanism for regulation of these changes may become clearer. The principal purposes of the proposed studies will be to determine the sequence of myosin isozyme transitions that occur from early embryonic development to adulthood and to determine if muscles of different physiological and biochemical type (fast and slow twitch) originate from single or multiple cell lineages during embryonic development. The specific objectives of these studies are: (a) To determine qualitatively and quantitatively the changes that do occur in myosin as particular anatomical muscles mature both in vivo and in vitro. These experiments will help resolve the question of the type of myosin isozyme transitions that appear normally in development of specific anatomic muscles in the embryo, neonate, and adult; (b) To determine if all types of muscle found in the adult originate from a single cell type suggesting that changing environmental cues during development and maturation are responsible for the sequential appearance of myosin isozymes; and (c) To determine the importance of rates of contraction on promoting synthesis of particular myosin isozymes and muscle cell "maturation". The latter will be accomplished by the use of a model in vitro system in which electrical simulation simulates the functional effect of innervation on myosin synthesis. Qualitative and quantitative analysis of myosin synthesis by skeletal muscle cells of the chick differentiation in vivo and cell culture will be done by column chromatography, one and two dimensional polyacrylamide gel electrophoresis, immunofluorescence, and immunoprecipitation.